The cross-bridge model proposes that the cross-bridge cycle of muscle is driven by the myosin cross-bridge alternating between two major conformations which differ markedly in their strength of binding to actin and in their overall structure. In the conformation, which occurs in the absence of nucleotide or the presence of ADP, myosin binds very tightly to actin at a 45 degrees angle. In the other conformation, which occurs when ATP or ADP.Pi is bound to myosin, myosin binds very weakly to actin at an angle postulated to be 90 degrees. These two conformations also differ in that the regulatory complex, troponin-tropomyosin, can greatly weaken the binding of the strong-binding conformation of nyosin.S-1 to actin, but has almost no effect on the binding of the weak-binding conformation of S-1. We previously found that the structure of acto.S-1 is very different in the presence and absence of of ATP, in agreement with our model. The structure of acto.S-1 was examined by negative staining using cross-linked actin.S-1, which enables actin to remain bound to S-1 at the low concentrations of protein needed for electron microscopy. In the present study, we examined the structure of cross-linked actin.S-1 in the presence of different ATP analogs to determine whether these analogs cause the conformation of acto.S-1 to resemble that obtained in ATP. We found that even though the ATP analogs, AMP-PNP and PPi, dissociate acto.S-1 to a similar extent, they, surprisingly, do not cause the same structural changes in acto.S-1. Cross-linked actin.S-1 in the presence of AMP-PNP appears quite rigor-like, whereas the structure of cross-linked actin.S-1 in the presence of PPi is ATP-like. We also examined the structure of pPDM modified S-1 when cross-linked to actin. Biochemical studies have shown that pPDM-modified S-1 resembles S-1.ATP in its interaction with actin both in the presence and absence of troponin-tropomyosin. In support of these biochemical studies, the structure of cross-linked actin.pPDM-modified S-1 in the absence of ATP resembles that of cross-linked actin.S-1 in the presence of ATP.